1. Technical Field of the Invention
The present invention relates to cellular telephone systems, and, in particular, to a method for managing the use of frequencies allocated to a cell.
2. Description of Related Art
Cellular telephone systems divide a large service area into a number of smaller discrete geographical areas called xe2x80x9ccellsxe2x80x9d each typically ranging in size from about one-half to about twenty kilometers in diameter. Each cell is contiguous with multiple adjacent cells to provide continuous coverage throughout the service area. A base station including a plurality of transceivers (i.e., channel equipment) capable of operating independently on different radio frequencies is provided for each of the cells. Via the transceivers, the base stations engage in simultaneous communications with plural mobile stations operating within the area of the associated cell. The base stations further communicate via data links (and voice trunks) with a central control station, commonly referred to as a mobile switching center, which functions to selectively connect telephone calls to and from the mobile stations through the base stations and, in general, control operation of the system.
Each cell is allocated the use of a predetermined set of frequencies, wherein each frequency comprises a physical channel supporting a plurality of logical channels (i.e., timeslots) therein. The availability of multiple frequencies per cell, with multiple logical channels per frequency, permits base stations to simultaneously handle many telephone conversations with many mobile stations. The frequencies allocated to a cell are preferably spaced apart across the frequency spectrum of the cellular band such that adjacent cells are not assigned close frequencies. This serves to minimize the instances of adjacent channel interference.
Because only a limited number of frequencies are available in the cellular band, an allocation of the same frequencies is repeated (i.e., reused) in other cells in a distant part of large service areas with many cells. No adjacent cells, however, are allocated the same frequency. Furthermore, the power levels of the signal transmissions on any given frequency are limited in strength. The foregoing precautions serve to minimize the likelihood of co-channel interference caused by reuse of that same frequency in a distant cell.
Although each cell is allocated certain specific frequencies and those specific frequencies are reused in a distant part of a large service area, it has been shown that interference may increase to the point of degrading quality when the frequency reuse plan is changed from, for instance, a 7/21 reuse to a 4/12 reuse, without over-dimensioning the transceivers. Traditional channel selection techniques for selecting the traffic channels for mobile stations use, such as interference driven channel selection (IDCS), are unable to overcome the quality degradation of reduced frequency reuse plan cellular systems. In order to improve the quality, the idea of creating virtual frequencies has developed to enable an increase in system capacity. The set of frequencies that are allocated to the cell are often referred to as xe2x80x9cvirtual frequenciesxe2x80x9d. The virtual frequency set includes assigned and unassigned frequencies. Assigned frequencies are those frequencies operating on available transceivers with a base station serving a cell. Unassigned frequencies are created by allocating more frequencies to a base station serving a cell than there are available transceivers to handle those frequencies.
One problem of the previously described telecommunications systems implementing interference driven channel selection is a degradation of signal quality when increasing capacity on the telecommunications system. Another problem is that the gain of current channel selection methods is decreased during periods of high traffic load. Without unassigned frequencies, there are fewer free frequencies to select among when assigning calls to traffic channels and the gain degrades using interference reducing methods, such as quality driven channel selection (QDCS) and interference driven channel selection (IDCS).
The present invention solves the problem of degradation of signal quality when increasing capacity on an interference driven channel selection/quality driven channel selection (IDCS/QDCS) equipped telecommunications system. The present invention improves signal quality by managing the frequencies allocated to a base station serving a cell to select the best of those allocated frequencies for assignment to base station transceivers. Measured quality metrics for certain ones of the allocated frequencies that are currently assigned to transceivers in the base station are compared against measured quality metrics for certain ones of the allocated frequencies that are currently unassigned to transceivers in the base station. Responsive to the results of the comparison, the best unassigned frequency is swapped for the worst assigned frequency.
The present invention provides a method for managing frequencies allocated to a cell within a cellular network to assign the best frequencies for use by channel equipment within that cell. The method measures at least one quality metric with respect to the allocated frequencies and compares the quality metrics for currently unassigned frequencies against the quality metrics for currently assigned frequencies. The best unassigned frequency is then swapped for the worst assigned frequency. Additionally, a voting step is used when comparing to indicate whether the unassigned frequency or the assigned frequency has a higher signal quality for communication.
Another aspect of the present invention is a method for assigning frequencies allocated to a base station serving a cell within a telecommunications network. At least one quality metric for at least two of the m frequencies allocated to the base station are measured. There are n of the m allocated frequencies currently assigned to the transceivers of the base station and mxe2x88x92n frequencies currently unassigned to the transceivers of the base station. A voting step occurs between the n assigned and the mxe2x88x92n unassigned frequencies based on the measured metrics to indicate whether the currently assigned or currently unassigned frequency is of better quality. The best mxe2x88x92n unassigned frequency is then swapped for the worst n assigned frequency in response to a positive vote for that particular unassigned frequency. Additionally, a step of comparing at least one metric of the n assigned frequencies to the mxe2x88x92n unassigned frequencies is performed.
Another aspect of the present invention is a system for arranging a set of frequencies allocated to a base station serving a cell within a telecommunications system. The system has a measuring device to measure at least one quality metric for a frequency currently assigned to a transceiver of a base station and at least one quality metric for a frequency not currently assigned to a transceiver. A processor operates to compare the measured quality metrics and exchange a certain one of the currently unassigned frequencies for a certain one of the currently assigned frequencies based upon the relative difference of the quality metrics. Additionally, the processor operates to vote, which adds a numeric value to a memory location based upon the results of the comparison of the quality metrics. A filter within the processor may also be included to prevent the swapping operation from occurring before a particular event occurs.